The present invention concerns a new stopper rod for regulating the flow of a molten metal from a casting group, for example, steel or cast iron, from a distributor or a casting ladle, and more particularly a one-piece stopper rod that has means for attachment to a lifting mechanism. In a particular embodiment, the stopper rod also has means for introducing an inert gas, such as argon, into the molten metal bath during continuous casting operations.
Such a stopper rod and its use are well known to the skilled artisan, in particular by U.S. Pat. Nos. 4,946,083 and 5,024,422, which are hereby incorporated by reference. These documents describe a one-piece stopper rod that can be attached to a lifting mechanism, comprised of:
a) an elongated body of refractory material that has a borehole positioned coaxially with respect to the body of the stopper rod and adapted to fixedly receive a metal rod for its attachment to a lifting mechanism. The axial borehole of the body of refractory material has an enlarged part with an annular sealing surface spaced away from the upper end of the body of refractory material. The means for attachment of the metal rod are generally positioned between the enlarged part and the lower end of the body of refractory material. At its lower end, the body of refractory material may have means for introducing gas into the molten metal bath; and
b) an elongated metal rod attached to the body of refractory material and having an axial borehole communicating in its lower part with the borehole of the body of refractory material. The rod has a collar carrying an annular sealing surface facing the annular sealing surface of the body of refractory material to create a gas tight seal. The upper end of the rod is adapted to be attached to a lifting mechanism that permits the vertical displacement of the stopper rod inside of a casting group such as a distributor. Means for fastening to the body of refractory material are generally positioned between the collar and the lower end of the metal rod.
The stopper rod may be connected to a gas supply line, generally but not obligatorily, through the upper end of the rod. The stopper rod may then permit the introduction of gas into the molten metal. When such a stopper rod is used, the gas is conveyed through the axial borehole of the body of refractory material to the stopper rod""s lower part. The body of refractory material may have in its lower part a means for introducing gas into the molten metal bath. The annular sealing surfaces of the rod and the body of refractory material facing each other and prevent substantial losses of inert gas and the infiltration of air.
To improve this tightness even more, it was proposed to place an annular gas tight gasket between these sealing surfaces. U.S. Pat. No. 4,946,083, for example, indicates that when a gasket with a thickness of about 0.4 mm and of a material resistant to high temperatures, e.g., graphite, is in place, the interface between the annular sealing surfaces of the rod and the body of refractory material furnishes a tightness capable of resisting a pressure up to 3 bars.
This seal is essential for casting high-grade molten metal. In the first place, it is necessary to assure a good protection against the infiltration of air responsible for oxidizing the molten metal during pouring. On the other hand, when an inert gas is injected through the stopper rod, it is also indispensable to reduce the losses of inert gas that cause production cost overruns, which are far from negligible.
Present systems still do not, however, furnish a completely satisfactory solution for these two points of view.
In pursuing their research in this domain, the applicants discovered that these problems are due to the fact that, for different reasons (unscrewing of the rod, expansion of the rod, etc.), a loss of tightness could occur at the sealing joint between the annular sealing surfaces of the rod and the body of refractory material facing each other.
The applicants then discovered that the tightness of the stopper rod could be improved by using a body of refractory material of a particular type.
According to the present invention, a stopper has a one-piece body of refractory material, constituted at least partially of a refractory material that is relatively impermeable to gases.